1. Field of the Invention
The present invention relates to improvements in the performance of implantable defibrillators, implantable cardioverter-defibrillators (ICDs) and other battery powered medical devices designed to provide high energy electrical stimulation of body tissue for therapeutic purposes.
2. Description of Prior Art
High energy battery powered medical devices, such as implantable defibrillators and ICDs, are designed to produce a strong electrical shock to the heart when called upon to correct the onset of tachyarrhythmia. The shock is produced by one or more energy storage capacitors that have been charged to a high voltage by the device's battery power source. The power source is typically a lithium/silver vanadium oxide (Li/SVO) battery or cell of the type disclosed in U.S. Pat. No. 5,458,997 of Crespi, and references cited therein. Crespi notes that the Li/SVO chemistry is useful for defibrillation applications because of its ability to produce pulses of energy that can charge the high voltage capacitors within the short time frame required by the device. In particular, the Li/SVO battery is typically called upon to charge the capacitors to deliver within 10 seconds or less a shock of up to 40 Joules. This must be done several times in succession if additional shocks are required. Unfortunately, as noted by Crespi, a Li/SVO cell can experience unpredictable resistance increase upon long-term discharge service. In particular, Li/SVO cells commonly have a two-stage run down with slightly different voltage plateaus at each stage. It is at the interval between the two plateaus where it is common to see the resistance increase described by Crespi. The problem is further explained in U.S. Pat. No. 6,426,628 of Palm et al. as being a transient phenomenon that occurs following a period of low current draw. When a load is reapplied (e.g., a defibrillation pulse is required), the resistance build-up temporarily prevents the cell from developing its full open circuit voltage potential. This condition, which is referred to as “voltage delay,” continues for a brief period until the resistance diminishes back to some nominal level.
In many cases, the voltage delay experienced by a Li/SVO cell is significant enough to impair the cell's ability to charge the capacitors of a defibrillator or ICD in a timely manner. This may result, prematurely, in a decision being made that the Li/SVO cell has reached end of service (EOS) and needs to be explanted for replacement. In addition to the patient inconvenience and risk entailed by the explantation procedure, a significant portion of the capacity of the Li/SVO cell is needlessly rendered unavailable for long-term use. Even if it is not removed, the cell's operation is unpredictable, thus making any attempt to calculate the EOS point rather complicated.
Additional shortcomings in the application of Li/SVO cells have been previously identified in U.S. Pat. No. 5,674,248 of Kroll et al. A typical Li/SVO cell suitable for the described applications has an energy density of about 0.4 watt-hours per kilogram (Wh/kg) as compared to 0.8 Wh/kg for lithium/carbon monofluoride (Li/CFx) cells and 0.9 Wh/kg for lithium/iodide (Li/I) cells, the latter being used almost exclusively for implantable pacemakers. This energy density disadvantage requires the use of a battery with greater volume and weight than would otherwise be needed if the Li/CFx or Li/I cells could be used. However, the Li/CFx and Li/I cells are unsuitable for the described applications because they cannot support the rapid discharge rates required for charging the defibrillator capacitors. The Li/SVO cell has the added disadvantage of significantly higher cost when compared to the Li/CFx and Li/I chemistries.
The Kroll et al. patent propose a staged energy concentration system for providing improved energy sources and device performance. A first energy stage utilizes a Li/CFx or Li/I battery to implement a primary power source. The first energy stage provides power to a second energy stage that utilizes either a lithium-based rechargeable secondary battery or a high energy density capacitor system. Energy is transferred at a low-rate from the first energy stage comprising the primary battery to the second energy stage comprising the rechargeable battery or the high energy density capacitor system. A trickle charge control circuit and a voltage doubler circuit are alternatively shown being interposed between the first and second energy stages. The second energy stage is rapidly discharged upon the detection of fibrillation to develop the high voltage charge needed for defibrillation therapy.
A shortcoming of the Kroll et al. high energy density capacitor system is the volume and number of capacitors needed for the second stage to support the storage of energy required, typically 200–300 Joules for a series of five therapeutic countershocks which might be required in a span of less than one minute. A shortcoming of the rechargeable battery system is that the lithium-based battery chemistries proposed for the secondary energy stage are not all suitable for the proposed application. Table 1 below sets forth the proposed chemistries. One is an LiMnO2 system, but this is a primary system and is not suited to recharging. Another is an LiSO2 system, but this operates with a sealed cell at a pressure of 3 to 6 atmospheres and is not suited for high-rate discharge applications. The remaining identified chemistries, namely LiMoS2, LiV2O5, LiTiS2, LiV6O13, LiCuC12, NiCad, Alkaline and Lead acid, have not found wide acceptance in the implantable device market.
TABLE 1Second Stage Candidate Cells Identified in U.S. Pat. No. 5,674,248ChemistryCell Voltage (VDC)LiMoS21.85LiMnO23.0LiV2O52.8LiTiS22.2LiV6O132.3LiCuC123.2LiSO23.1
A broader shortcoming is the failure of the Kroll et al. patent to identify a specific selection for a first stage battery and a configuration for the identified trickle charge control circuit or the voltage doubler circuit. A Li/I battery with a beginning of life open circuit voltage of 2.8 volts DC is identified within the disclosure as a potential candidate for the first energy stage. The output voltage of this cell falls to about 2.6 volts at EOS so the cell would be incapable of charging LiMnO2, LiV2O5, LiCuC12 and LiSO2 cells unless the trickle charge control circuit utilized a means of increasing the first stage output voltage. This is clear because each of these cells has a higher operating voltage than the Li/I battery. The voltage doubler circuit is used in conjunction a first stage primary battery continuously recharging a second stage rechargeable battery. The Kroll et al. disclosure does not identify a method or means of controlling the flow of energy while recharging that is necessary to prevent damage to or catastrophic failure of the second stage cell or cells.
A need therefore exists for improvement in defibrillator/ICD battery power systems so as to overcome the above-described deficiencies of the prior art.